1. Field of the Invention
The present invention relates to pressure gauges, and, more particularly, to pressure gauges for blood pressure measuring devices, or so-called sphygmomanometers, which are designed to visually indicate the air pressure in a blood pressure measuring cuff which is attached to a limb of the person whose blood pressure is to be determined.
2. Description of the Prior Art
The measurement of a person's blood pressure, as proposed by Riva-Rocci and Korotkoff, involves the attachment of an inflatable measuring cuff to an extremity of the person under examination and the application of a pressure to that cuff, from a suitable pressure source, to a pressure level which is higher than the anticipated systolic blood pressure value. Using an adjustable deflation valve, the air pressure in the measuring cuff is then gradually reduced, while the blood circulation under the cuff is monitored through auscultation. Such auscultation reveals that, at a certain pressure in the measuring cuff, the blood circulation starts to produce characteristic rushing sounds, the so-called Korotkoff noises. This pressure level represents the systolic blood pressure. As the pressure in the measuring cuff is further lowered, it reaches a level at which these blood circulation noises disappear again. This second pressure level represents the diastolic blood pressure. This method of blood pressure examination has received widespread acceptance, because of its absence of risk to the person examined, its extreme simplicity, and its low cost.
The determination of the systolic and diastolic blood pressure levels is commonly performed visually, using a simple air pressure gauge which is connected to the measuring cuff or to the pressure hose which leads to the cuff. The prior art in this field discloses various pressure gauges for this purpose, a change in air pressure being typically converted into an angular movement of a pointer over a circular dial. Typically, the pressure gauge utilizes a flexible pressure capsule whose interior space communicates with the measuring cuff. The pressure capsule has one wall which is flexible so as to bulge outwardly, as the pressure inside the capsule increases. A suitable mechanism is used to pick up and magnify these displacements of the capsule wall, transmitting them to a spring-biased rotatable shaft which carries a pointer. This pickup and magnification mechanism includes a sensing member on a rotatably supported sensing shaft, the sensing member being a wire part, curved like a skid and bearing against the flexible wall of the pressure capsule under a spring bias. The sensing member thus transmits the expansion and contraction displacements of the pressure capsule to the sensing shaft in the form of angular displacements of the latter. A long transmission pin, extending radially from the sensing shaft and in a direction away from the pressure capsule, then transmits these displacements in magnified values to a swivel arm which is arranged in its path and supported on a pivot shaft. The latter is oriented perpendicularly to both the sensing shaft and the flexible pressure capsule wall. The far extremity of the swivel arm carries a gear segment in meshing engagement with a pinion which is mounted on the rotatably supported pointer shaft. The latter, in addition to carrying the pointer itself, also carries a spiral spring which provides the spring bias between the sensing member and the flexible wall of the pressure capsule, thereby also eliminating any transmission play.
Most sphygmomanometer pressure gauges use a circular pressure capsule whose flexible wall is a diaphragm-type wall, i.e. a large wall portion which is surrounded by circular undulations in the capsule wall. The expansion and contraction displacements of this diaphragm wall are substantially proportional to the pressure changes inside the pressure capsule. The pickup and magnification mechanism converts these displacements into much larger angular displacements of the gauge pointer. However, the two lever-type force transmissions in the magnification mechanism produce certain distortions in the transmission ratio, due to changes in the effective lengths of the lever arms, so that the displacement values shown by the pointer are no longer exactly linear in relation to the actual pressure values. Fixed pressure increments in a higher pressure range thus produce smaller pointer increments, meaning that the markings on the gauge dial have to be more closely spaced for higher pressure values. Generally, it is not a great disadvantage to have such non-linear dial markings on the pressure gauge, provided the pressure range distribution on the gauge dial remains unchanged, i.e. when the zero-position on the dial is a fixed one. To the extent that there exist small variations in this zero-position, as a result of assembly and manufacturing tolerances, these variations are negligible, in comparison to other variables which affect the accuracy of this kind of blood pressure measuring procedure.
The blood pressure values obtained in accordance with the above-described auscultatory measuring approach, as suggested by Riva-Rocci and Korotkoff, when compared to actual blood pressure values obtained through a "bloody measurement" taken inside an artery of the patient, are subject to considerable deviations from these actual pressure values, depending upon the structure of the limb on which the measurements are taken. Commonly, the auscultatory blood pressure measurements are taken on the left upper arm of a person. If that person is of a slight build, with a minimum of soft body tissue on the upper arm, for example, or if that person is of a rather heavy build, with considerable muscle and/or fatty tissue at the point of measurement, the measurements obtained by the auscultatory method may deviate from the actual blood pressure values by as much as 25 mm Hg in either direction, respectively. It has been found that the correction factors which have to be applied to the systolic and diastolic pressure values, as a result of different structures of the patient's limb, are approximately proportional to the circumference of that limb, so that a compensatory correction to obtain the actual values requires only a simple angular shift of the zero-position on the pressure gauge dial. Such a zero-position shift, however, makes it a prerequisite that the gauge dial has identical linear increments over its entire measurement range. By the same token, it follows that a zero-position shift is not possible with the above-described prior art pressure gauge.